1. Field of the Invention
The present invention relates to a BTL (balanced transformer-less or bridge-tied load) amplifier, in particular, relates to a BTL amplifier composed of semiconductor integrated circuits.
2. Description of the Related Art
A BTL amplifier has been generally in use for a power amplifier for an electronic device of relatively high power consumption such as an audio equipment, an audio output device of a television receiver, or the like. The BTL amplifier includes two output circuits, each having a single output, and drives those circuits in an opposite phase in relation to each other. A load such as a speaker or the like is connected between output terminals of the output circuits. This causes the amplitude of an individual voltage output from each output circuit to be half of the amplitude of a total voltage to be applied to the load. Allowing a half voltage amplitude means lowering power consumption of each output circuit. As described above, since a withstand voltage or a current capacity of a circuit element of the BTL amplifier is smaller than that of an amplifier having a single output, the BTL amplifier facilitates miniaturization and power saving. These features particularly have an advantage in circuit integration.
In recent years, the BTL amplifier is frequently in use for a portable information-processing device such as a mobile phone or the like due to the above-mentioned features.
FIG. 6 is one example of a plan view of an output transistor included in a BTL amplifier of a prior art (see the Japanese patent laid-open publications No. 61-142763 and 2-132838.)
An output transistor 100 of this BTL amplifier is mounted in a rectangular region on a substrate. The rectangular region is divided into four rectangular parts 10, 11, 12 and 13 in lattice patterns. Each of the rectangular parts 10, 11, 12 and 13 is equivalent to a single transistor. The rectangular parts 10, 11, 12 and 13 are hereinafter referred to as transistor parts.
FIG. 7 is a plan view showing a layout of three semiconductor regions 102, 104 and 106 for a transistor part 10 of FIG. 6 included in the BTL amplifier of the prior art. In each of transistor parts 10, 11, 12 and 13, as shown in FIG. 7, semiconductor regions 102, 104 and 106 which are laterally parallel to each other are located repeatedly in a longitudinal direction. For example, the semiconductor regions 102, 104 and 106 are n-type semiconductor region, n-type semiconductor region and p-type semiconductor region, and compose a collector region, an emitter region, and a base region of an NPN transistor, respectively. Contacts 103 and 105 are located in the semiconductor regions 102 and 104, respectively.
As shown in FIG. 6, a power supply terminal 1 is provided at an edge of the transistor part 10. The power supply terminal 1 is connected to an external constant voltage source (not shown) and is kept at a constant potential. A ground terminal 2 is provided at an edge between the transistor parts 11 and 13. Output terminals 3 and 4 are provided at each edge of the transistor parts 11 and 13. An external load such as a speaker or the like is connected between the output terminals 3 and 4.
The four transistor parts 10, 11, 12 and 13 are covered with four wires 51, 52, 53 and 54.
These wires 51, 52, 53 and 54 are preferably a metal wire layer made of aluminum or the like and composed by a multi-layer wiring process. The four transistor parts 10, 11, 12 and 13 are connected to the power supply terminal 1, the ground terminal 2, and the two output terminals 3 and 4 through the four wires 51, 52, 53 and 54, as follows.
In the transistor parts 10 and 12 which are laterally adjacent to each other, the collector region 102 is connected to the wire 51 through the contact 103. The wire 51 is connected to the power supply terminal 1. In the transistor parts 11 and 13 which are laterally adjacent to each other, the emitter region 104 is connected to the wire 52 through the contact 105. The wire 52 is connected to the ground terminal 2. The emitter region 104 of the transistor part 10 is connected to the wire 53 through the contact 105, and the collector region 102 of the transistor part 11 is connected to the wire 53 through the contact 103.
The wire 53 is connected to the output terminal 3. The emitter region 104 of the transistor part 12 is connected to the wire 54 through the contact 105, and the collector region 102 of the transistor part 13 is connected to the wire 54 through the contact 103. The wire 54 is connected to the output terminal 4.
Therefore, the transistor parts 10 and 11 are equivalent to two transistors connected in series across the power supply terminal 1 and the ground terminal 2, and constitute one push-pull type output circuit. In a similar way, the transistor parts 12 and 13 are equivalent to two transistors connected in series across the power supply terminal 1 and the ground terminal 2, and constitute another push-pull type output circuit.
The base regions 106 of the four transistor parts 10, 11, 12 and 13 are connected to external preamplifiers (not shown) through other wires which are different from the four wires 51, 52, 53 and 54. A push-pull operation for the transistor parts 12 and 13 and that for the transistor parts 10 and 11 are controlled in opposite phase to each other through the preamplifiers. That is, two push-pull type output circuits are driven in opposite phase to each other. As a result, voltage amplitude of an output voltage output across the two output terminals 3 and 4 is twice as large as amplitude of individual electrical potential change of respective output terminals 3 and 4.
For further improved integration of the transistor parts 10, 11, 12 and 13, further miniaturization of the wires 51, 52, 53 and 54 is required.
However, in the BTL amplifier of the prior art, positions to locate the power supply terminal 1, the ground terminal 2, and the two output terminals 3 and 4 for the transistor parts 10, 11, 12 and 13 are limited to positions shown in FIG. 6, for example. In particular, the power supply terminal 1 is located apart as far as it can be from the other terminals 2, 3 and 4 to prevent short-circuiting between the power supply terminal 1 and the other terminals 2, 3 and 4. On the other hand, the wires 53 and 54 extend linearly in parallel to longitudinal direction of the transistor parts 10, 11, 12 and 13, namely in a direction perpendicular to the lateral semiconductor regions 102, 104 and 106 included in the respective transistor parts 10, 11, 12 and 13, or in a longitudinal direction in FIG. 6. As a result, the wire 51 has to protrude outside of the transistor parts 10 and 12 so as to keep the wire 51 apart as far as it can be from both the wires 53 and 54 and prevent short-circuiting between the wire 51 and the others. That is, the wire 51 has to be longer than the other wires. The longer the wires are, the larger the wire resistances become. Therefore, it is difficult to further improve integration degree with keeping wire resistance low. The above-mentioned wire resistance rise is not preferable since the wire resistance rise increases power loss and reduces power to be output.